This is a problem that goes all the way to the core. It isn't very good OO (some will argue PHP isn't really OO anyway) because core types are not objects. Whether you have substr(..., ...), Helper::substr(..., ...) or getSomeInstance()->substr(..., ...) it's really all the same, with maybe some added benefits if you provide some wrapper functionality as in the last case.

   $username = InputSanitizer::getInstance()->getPostData('username');

There. Now it's OO ;P.

I would rather use Java in that case.

Your wish is my command.

    $username = array();
    exec('java sanitize.class ' . escapeshellarg($_POST['username']), $username);

And very fortunately so. 'Pure' OO is the madness that created Java.

Encapsulation is overrated. So overrated that you need another fancy concept like 'dependency injection' when it just gets in the way.

The static function is simple and because of that, it is good.

tabbyjabby didn't say it's bad. They just said it's not OO.

I know. It implies the assumption that not complying with OO is something code should avoid as a rule of thumb.

I'm the one saying that strict OO is bad because it's verbose and its slower than the alternative.

I think it more implies that object oriented things should be called OO and not-object-oriented things should be called something that isn't OO.

Count me in as saying that not only strict OO is bad, but OO is a bad idea in general. At least in its usual form. You might manage to tackle something OO-like on, say, FP, and get away with it.